NFS: teach nfs_lookup_verify_inode to handle LOOKUP_RCU

It fails with -ECHILD rather than make an RPC call.

This allows nfs_lookup_revalidate to call it in RCU-walk mode.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/dir.c

@@ -1050,6 +1050,8 @@ int nfs_lookup_verify_inode(struct inode *inode, unsigned int flags)
 out:
 	return (inode->i_nlink == 0) ? -ENOENT : 0;
 out_force:
+	if (flags & LOOKUP_RCU)
+		return -ECHILD;
 	ret = __nfs_revalidate_inode(server, inode);
 	if (ret != 0)
 		return ret;
@@ -1135,11 +1137,11 @@ static int nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
 	if (!nfs_is_exclusive_create(dir, flags) &&
 	    nfs_check_verifier(dir, dentry, flags & LOOKUP_RCU)) {
 
+		if (nfs_lookup_verify_inode(inode, flags)) {
 			if (flags & LOOKUP_RCU)
 				return -ECHILD;
-
-		if (nfs_lookup_verify_inode(inode, flags))
 			goto out_zap_parent;
+		}
 		goto out_valid;
 	}